Use CF_EXPOSED_USERNAME@example.com / CF_EXPOSED_PASSWORD
    <form method="POST">
      <label for="login">Username:</label>
      <input type="text" id="login" name="username" required><br>

      <label for="password">Password:</label>
      <input type="password" id="password" name="password" required><br>

      <input type="submit" value="Login">
    </form>